This invention relates generally to borehole formation evaluation instrumentation. More particularly, this invention relates to a new and improved calibration system for use in an electromagnetic propagation based borehole formation evaluation instrument used primarily in oil and gas well drilling applications.
Borehole formation evaluation tools are known which measure phase and/or amplitude of electromagnetic waves to determine an electrical property (such as resistivity or permittivity) of a section of a borehole. Typically, the existing tools used for this application are composed of one or more transmitting antennas spaced from one or more pairs of receiving antennas. An electromagnetic wave is propagated from the transmitting antenna into the formation surrounding the borehole and is detected as it passes by the two receiving antennas. In a resistivity measuring tool, magnetic dipoles are employed which operate in the mf and lower hf spectrum. In contrast, permittivity tools utilize electric dipoles in the VHF or UHF ranges.
In a known resistivity sensor of the type hereinabove discussed which is used by Teleco Oilfield Services Inc., assignee of the present application, the resistivity sensor measures both phase difference and amplitude ratio to provide two resistivities with different depths of investigation. A signal received in a first receiving antenna is shifted in phase and its amplitude will be less than the signal received in a second receiving antenna. Resistivities are then derived from both the phase difference and the amplitude ratio of the received signals. This differential measurement is primarily responsive to the formation opposite the receiving antennas and is less sensitive to the borehole and/or variations in the transmitted signal as in prior art sensing devices.
While well suited for its intended purposes, a problem with existing electromagnetic propagation sensors of the type described herein consists of measurement uncertainty introduced by phase and amplitude differentials between the two measurement channels. A measurement channel includes a receiving antenna, a radio receiver and such signal processing means as may be necessary to determine the phase and amplitude of the received signal relative to the other channel or a known reference. Phase and amplitude differentials can occur with antenna deformation caused by borehole temperature and pressure variations, antenna insulator deterioration or thermal drift of the electronic components.
Such phase and amplitude differentials become more difficult to compensate through ordinary means (such as increased mechanical precision and ruggedness or electronic component matching) as the measurement bandwidth is reduced. This is because narrow bandwidth channels are more sensitive to thermal and mechanical changes than are wide bandwidth channels. As a result, efforts to improve measurement resolution (i.e., the ability to log small changes in the formation characteristics) by decreasing the system bandwidth are frustrated by the reduced overall accuracy introduced by phase and amplitude differentials. One problematic manifestation of this differential effect is "divergence" of the phase and amplitude logs of a borehole section. Such "divergence" reduces the confidence of the formation evaluation and thereby forces the allowance of a wide error band in the log interpretation.
One prior art solution to the phase and amplitude differential problem is the Compensated Dual Resistivity sensor. This device employs transmitting antennas at each end of the drill collar, with a pair of receiving antennas located centrally between them. By firing one transmitting antenna and then the other, the two differential measurements will have equal and opposite error components. Averaging the two measurements cancels the errors. This type of sensor is by necessity about one meter longer than the analogous electromagnetic propagation type tool used by Teleco Oilfield Services, Inc. There are a number of disadvantages associated with this greater length. Significantly, higher drill collar weight, higher material and machining costs, more difficult transportation, storage and handling considerations, as well as a somewhat reduced immunity to invasion effects. This is because the measurement occurs farther from the bit. In addition, at slow penetration rates, drilling mud can invade the formation before the sensor can log it.